Electric power systems have different types of capacitive loads due to physical characteristics of transmission or distribution lines or due to the addition of capacitors for serial or shunt compensation or for filtering of harmonics. Serial or shunt compensation is used to maintain a constant voltage profile along the distribution or transmission lines. These capacitive loads may be switched on or off, as needed, with a circuit-breaker.
When a capacitor bank is de-energized, some residual DC voltage charges remain in its electrostatic field. As a capacitor bank has self-discharging characteristics, the voltage charges level is dynamic and is decreasing in time from the de-energization to the complete capacitor bank discharge. A capacitor bank may take up to 15 minutes to discharge itself after the breaker opening.
In order to reduce or eliminate the voltage transients such as inrush current caused by a capacitor bank energization, the optimal moment to switch on a discharged capacitor is at the zero crossing of the voltage on the waveform. To perform this operation at the exact moment, a CSD is needed. In normal operation conditions in a typical electrical network, capacitor banks may be switched several times a day.
Many applications require the use of capacitive load switching. For example, distributed energy resources (DERs) integration, static VAR compensator (SVC) and static synchronous compensator (STATCOM) systems may switch capacitor banks several times per day to achieve voltage regulation. However, with current technologies, when capacitor banks are de-energized, a power system or DER operator needs to wait until the complete discharge of the capacitors before energizing them again in order to avoid the inrush current on the network. The inrush current (2), shown in FIG. 1, illustrates the result of an uncontrolled switching operation of a capacitive load on the network. The consequences of such inrush current may be critical as they may damage the equipment, hence reducing the reliability of the system. In this example, the voltage (1) is also illustrated. Furthermore, the resulting transients may travel on the lines and trip a protection reducing then the stability of the network and potentially leading into a major blackout.
Current technologies reduce the inrush current on capacitor banks by using either pre-insertion resistors or CSDs. However, with such solutions, capacitor banks need to be discharged in order to avoid the inrush current when performing the switching operations. Furthermore, the 15 minute waiting time for capacitor discharge is problematic as it does not allow the power system and DER operators to switch at their convenience resulting in a non-optimal operation of their installations.
In many cases, a blocking timer is used to control the circuit breaker closing to avoid operation when capacitive loads are still charged. The timer is armed each time the breaker is opened thus blocking the closing of the breaker.
There is thus a need for a new technique to reduce the switching time of capacitive load while minimizing the inrush current.